Tunnels are generally of a few metres to several kilometers. Its diameter ranges from that of less than a metre to 16-17metres. It may be near to the ground or may be at large depths below the ground surface, such as those tunnels in mountains. Before starting the tunneling work (process of making a tunnel), following investigations should be carried out meticulously -

1. Hydrological survey


2. Geological survey and exploration


3. Rock temperature and gases


4. Factor influencing the depth of the tunnel below the ground level


5. Factor affecting the diameter of the tunnel


6. Geological features along the axis of the tunnel

The above factors are important for arriving at the design of the tunnel, but a site engineer has to investigate the parameters before design. Also, a construction engineer should always verify the available data for design. If there is any discrepancy in the existing profile with the coordination during design of a tunnel, modifications are needed.